Photochemically and Electrochemically Driven Homogeneous Catalysts for Hydrogen Oxidation and Carbon Dioxide Reduction

The development of renewable, carbon-neutral energy sources is compelled by the devastating environmental consequences that will result from the continued combustion of fossil fuels. Solar energy provides the capacity to easily meet our projected energy demand, but it must be converted to a form more easily stored and distributed. This can be achieved through artificial photosynthesis, which converts solar energy to chemical fuels. One attractive energy-storing artificial photosynthetic reaction is the light-driven reverse water-gas shift reaction (RWGS, CO2 + H2 → CO + H2O), which requires the successful marrying of photocatalytic hydrogen oxidation and carbon dioxide reduction. Prior work in our group elucidated a comprehensive kinetic and mechanistic picture of the photosensitization of a system capable for achieving the light-driven RWGS reaction, but greater understanding of catalytic H2 oxidation and CO2 reduction is required in order to develop a functionally competent system. Firstly, a molecular nickel hydrogen oxidation catalyst is studied through a combination of electron paramagnetic resonance techniques, X-ray crystallography, and density functional theory calculations. Photocatalytic hydrogen oxidation with the same catalyst is then detailed, with an initial stepwise approach determining that every process necessary for accomplishing catalysis is achievable with photochemical sensitization. Catalytic turnover is demonstrated in a photoelectrocatalytic system, with an electrode acting as an electron acceptor. Secondly, molecular catalysts for CO2 reduction are studied with the aim of understanding how catalytic activity is influenced by the nature of the monodentate ligands. Whether efficient catalysis requires the routinely employed but photolabile CO ligand is explored. The electrocatalytic reduction of CO2 by Ru–bipyridyl compounds is investigated and their visible-light photochemistry is also discussed. Finally, the development of a functional system for the light-driven RWGS reaction is discussed. The key design criteria for a system are elucidated and the use of promising nickel and ruthenium catalysts is probed. Electrochemical techniques are utilized to efficiently screen reaction conditions. Although key steps of the reaction are demonstrated, the photochemical experiments intended to demonstrate the light-driven RWGS reaction yielded no more than trace production of CO. Additional research is required to develop a fully functional system for the light-driven RWGS reaction.